Of the sources of electricity we have mentioned two: Friction, and Galvanism or chemical action. There are hundreds of forms of the latter species of apparatus for generating electrical energy, so we will mention only a few of the more prominent ones. It is not our intention to go into the chemistry of batteries. There are too many exhaustive works on this subject lying on the shelves of libraries that are accessible to all. All galvanic batteries act on one general principle—the generation of electricity by the chemical action of acid on metal plates; but the chemistry of their action is very different. In all batteries the potential energy of one element is greater than the other. The acid of the battery dissolves the element of greater potentiality, and its energy is freed and under right conditions takes on the form of electricity. The potential of zinc, for instance, is greater than that of copper, and the measure of the difference is called the "electromotive force," the unit of which is the "volt." Electromotive force is another name for pressure; the symbol for which is E.M.F.

If we were to put two zinc plates in the battery fluid and connect them in the ordinary way there would be no electricity evolved (assuming that they were perfectly homogeneous), because they are both of the same potential, or have the same possible amount of stored electrical energy measured by its working power. If one of the zinc plates were softer than the other, a feeble current would be developed, for one would be more readily acted upon by the acids than the other. The battery that has been most used in America for telegraphic purposes is called the gravity-battery. It is constructed by putting a copper plate in some form at the bottom of a jar, usually of glass, and filling it partly full of the crystals of sulphate of copper, commonly called "bluestone." Zinc, usually cast in some open form, so as to expose a large surface to the solution, is suspended in the upper part of the jar, which is then filled with water till it covers the zinc. The zinc is the positive metal, but it is called the negative pole. The energy developed by the zinc passes from zinc to copper and out on the circuit from the copper pole. Hence the copper came to be called the positive pole, although in relation to zinc it is negative. Copper would, however, be positive to some other metal whose potential was less. So you see that metals are relative, not absolute, in their character as positive and negative elements.

The galvanic battery has been almost entirely superseded in this country for telegraphic purposes by the dynamo, a machine developing electrical currents by mechanical power. Another form of battery that is extensively used for some kinds of heavy current work is called the storage-battery. The man who did the most, perhaps, to bring the storage-battery to its present state of perfection was PlantÉ, a Frenchman, who died only a short time ago. Although very many types of battery have been developed, it is found that, after all, the lines on which he developed it make the most efficient battery. There is a common notion that electricity is stored in the storage-battery. Energy is stored, that will produce electricity when it is set free, just the same as energy is stored in zinc. The storage-battery, when ready for action, is one form of acid or primary battery. It has been made by passing a current of electricity through it until the chemical relations of the two lead plates have been changed so that the potential of one is greater than that of the other. A simple storage-battery element is made up of two plates of lead held out of contact with each other by some insulating substance the same as the elements of an ordinary battery. The cell is filled with dilute sulphuric acid, and there will be no electrical action till the cell has been charged by running a current of electricity through it and forming a lead oxide on one plate. Now, take off the charging battery and connect the two poles, and electricity will flow until the oxide has partly changed back into spongy metallic lead, when it must be renewed by recharging.

I remember perfectly well the first galvanic battery I ever saw, for it was of my own construction. It is now nearly fifty years ago, and yet it seems but yesterday—such is the flight of time. I related to you in another chapter how I made a voltaic battery—or pile, as it was called—by cutting up my mother's boiler and her stove-zinc, and the domestic incident that followed. Well, a little later I made a real galvanic battery as follows: I lived in the country and far from town or city, and my facilities were extremely limited, so that I pursued my scientific investigations under great difficulties. My only text-book was an old Comstock's Philosophy. In the book was a crude cut of a Morse register and a short description of its construction, including the battery. I determined to make a register, and I did. It was all constructed of wood except the magnet and its armature and the embossing-point, which latter was made of the end of a nail. The thing that seemed out of reach was the electromagnet. I had no money; and there was no one that believed I could do it, and if I could "what good would come of it?" I made friends with a blacksmith by keeping flies off a horse while he nailed the shoes on, and "blowing the bellows" and occasionally using the "sledge" for him. When I thought the obligation had accumulated a sufficient "voltage" (to express it electrically) I communicated to the blacksmith the situation and what I wanted.

The good-natured old fellow was not long in bending up a U magnet of soft iron and forging out an armature. The next step was to wind the U with insulated wire. The only thing that I had ever seen of the kind was an iron wire called "bonnet" wire that was wrapped with cotton thread. This, however, was not available, so I captured a piece of brass bell-wire and wound strips of cotton cloth around it for insulation—and in that way completed the magnet.

Now everything was ready but the battery. I went at its construction with a feeling almost akin to awe, for I could not believe that it would do as described in the book. I procured a candy-jar from the grocer and found some pieces of sheet zinc and copper. These I rolled together into loose spirals and placed one inside the other so that they would not touch, when I was ready for the solution. The druggist trusted me for a half pound of "blue vitriol," and I put it into my battery and filled it with water. I waited awhile for it to dissolve, and then connected my magnet in circuit, when—to my astonishment and delight—it would lift a pound or more. It was a great triumph. I never have had one since that gave me the same satisfaction. But I had my triumph all to myself. I was still the same "tinker" (a name I had long carried), and a nuisance to be endured but not encouraged.

The dynamo is the form of generator now in general use where heavy currents of electricity are needed. It is aptly described by a writer in Modern Machinery, Mr. John A. Grier, as a thing that when "at rest is a lifeless piece of mechanism; in action it has a living spirit as full of mystery as the soul of man." This is a poetic way of describing it that conveys to the mind a sense of the power and beauty of natural law in action, that would not come from a mere recital of the cold scientific facts. The facts, however, are necessary: but let us draw from them all the poetry and all the practical lessons that we can as we go along; for it is this blending of the poetic with the practical that lends a charm to our every-day "grind," and lightens the load of many a weary hour.

The dynamo is a machine that converts mechanical into electrical energy, and the great practical value of energy in this form is that it can be distributed through a conductor economically for many miles. We can transmit mechanical power by means of a rope or cable for a limited distance, but at tremendous loss through friction. We can transmit power through pipes by compressed air or steam, but there is a great loss, especially in the case of steam, by condensation from cold. None of these methods are available for long distances. Another advantage electricity has over other forms of energy is the speed with which it can be transmitted from one place to another. In this respect it has no rival except light. But we have not been able to harness light and make it available to carry either freight or news, except in the latter case for a short distance by flashing it in agreed signals.

The heliostat can be used when the sun shines to transmit news by flashes of sunlight chopped up into the Morse code and thrown from point to point by a moving mirror. But this is limited as to distance; besides, the sun does not always shine. It has the disadvantage in that respect that the old semaphore-telegraph did that was in use in Wellington's day. These semaphores were constructed in various ways, but a common form was that of moving arms that could be seen from hill to hill or point to point. By a code of moving signals news was repeated from point to point and it can be easily imagined that many mistakes occurred, to say nothing of the time it required for repetition. When the battle of Waterloo was fought—so the story goes—news was sent to England by means of the semaphore-telegraph. The dispatch read, "Wellington defeated—" At that point in the message a thick fog came up and lasted for three days, so that no further news could be sent or received. In the telegraphic parlance of to-day the line was "busted." For three long days all London was in deep mourning, when finally the fog lifted, which repaired the telegraphic line, and the balance of the dispatch was received—"the French at Waterloo." Mourning changed to rejoicing and the English have rejoiced ever since when they think of either Wellington or Waterloo.

But to return to the dynamo. The name dynamo is an abbreviation for dynamo-electric machine. A machine for producing dynamic electricity. There are many forms of the dynamo, just as there are in the evolution of every important machine, and there will be many more. But the fundamental, underlying principle of them all is contained in an experiment made by Faraday. Faraday took the soft iron "keeper" of a permanent magnet and wound insulated wire around it and brought the two ends of the wire close together. He now placed the keeper, with the wire wound around it, across the poles of the permanent magnet, and wrenched it away suddenly, when he observed a spark pass between the ends of the wires. This would occur when he approached the poles as well as when he took it away. He discovered that the currents were momentary and occurred at the moment of approach or recession, and that the currents developed by the approach were of opposite polarity to those occurring at the recession. When the "keeper" was put on the poles of the magnet it was magnetized by having its molecular rings broken up and the poles of the little natural magnets all turned in one direction. During the time that the molecules of the keeper are changing they are in a dynamic or moving condition. By some mysterious action of the ether between the iron and the wire wrapped around it there is a corresponding molecular action in the wire that is dynamic for a moment only, and during that moment we have the phenomenon of an electric current. When the magnet and soft iron are separated this molecular state of strain is relieved and the molecules of both the iron and the wire wound about it return to normal, and in the act of returning we have a dynamic or moving condition, resulting in a current, only in the opposite direction. (See Chap. VI.)

Now mount the permanent magnet in a frame and mount the soft iron with the wire on it (which in this shape is an electromagnet) on a revolving arm and so set it on the arm that its ends will come close to, but not touch, the poles of the permanent magnet. Now revolve the arm, and every time the electromagnet or keeper approaches the permanent magnet a current of one polarity will be momentarily developed in the wire of the electromagnet, which is moving. When it is opposite the poles, it has reached the maximum charge and, now, as it passes on it discharges and a current of the opposite polarity is developed in the wire. The more rapidly we revolve the arm the more voltage (electrical pressure) the current it develops will have.

It will be plain to all that we might make the electromagnet stationary and revolve the permanent magnet and get the same result. If the permanent magnet were strong enough and the electromagnet the right size as to iron, windings, etc., and we revolve the arm with sufficient rapidity, we could get an alternating current of electricity that would produce an electric light. I have not and cannot here give you the construction of a modern alternating-current dynamo. I have simply described the simplest form of dynamo, and all of them operate upon the fundamental principle of a permanent magnetic field and an electromagnet, moving in a certain relation to each other. The field may revolve or the electromagnet may revolve, whichever is the most convenient to construct. The field-magnet may be a permanent magnet or an electromagnet, made permanent during the operation of the dynamo by a part of the current generated by the machine being directed through a coil surrounding soft iron; or the field-current may come from an outside source. This is the kind of field-magnet universally used for dynamo work, as a much stronger magnetism is developed in this way than it is possible to obtain from any system of permanent steel magnets.

The usual construction is to have a stationary field-magnet and then a series of electromagnets mounted and revolving upon a shaft in the center of the magnetic field. The rotating part is called the armature, and is so wound with insulated wire that successive induced currents are created in the armature windings and discharged through brushes which rest on revolving segments that connect with the armature windings. These induced currents succeed each other with such rapidity as to amount in practice to a steady current. However, the separate pulsations are easily heard in any telephone when the circuit is near to that of a dynamo circuit. The dynamo current is not nearly so steady as the battery current, although both are probably made up of separate discharges. In the dynamo there is a discharge every time the electromagnet of the armature cuts through the lines of force of the magnetic field, and in the galvanic battery every time a molecule is broken up and its little measure of energy is set free. In the dynamo the pulsations are so far apart as to make a musical tone of not very high pitch, but in the galvanic battery the pitch of the tone, if there is one, would require a special ear to hear it—one tuned, it may be, up near the rate of light vibration.

There are two types of dynamo, one generating a direct and the other an alternating current. (By alternating we mean first a positive and then a negative current impulse.) We cannot enter into a technical description of the dynamo in a popular treatise such as this.

The dynamo has evolved from the germ discovered by Faraday, till to-day it is a machine, the construction of which requires the highest class of engineering skill. When in action it seems like a great living presence, scattering its energy in every direction in a way that is at once a marvel and a blessing to mankind. But we must not give all the credit to the dynamo. As the moon shines with a reflected light, so the dynamo gives off energy by a power delegated to it by the steam-engine that rotates it, and the steam-engine owes its life to the burning coal, and the burning coal is only giving up an energy that was stored ages ago by the magic of the sunbeam; and the sun—? Well, we are getting close on to the borders of theology, and being only scientists we had better stop with the sun.

There is still another way of generating electricity besides those that we have named; which are friction, chemical action, and the magneto-electric mode of generating a current. Electricity may be generated by heat. If we connect antimony and bismuth bars together and apply heat at the junction of the metals and then connect the free ends of the two bars to a galvanometer, it will indicate a current. These pairs can be multiplied, and in this way increase the voltage or pressure, and, of course, increase the current, if we assume that there is resistance in the circuit to be overcome. If there were absolutely no resistance in the circuit—a condition we never find—there would be no advantage in adding on elements in series.

Substances differ in their resistance to the passage of electricity—the less the resistance the better the conductor. The German electrician, G. S. Ohm (1789-1854), investigated this and propounded a law upon which the unit for resistances is based, and this unit takes his name and is called the "ohm."

Any two metals having a difference of potential will give the phenomena of thermo-electricity. Antimony and bismuth having a great difference of potential are commonly used. The use made of thermal currents is chiefly for determining slight differences of temperature. An apparatus called the thermo-electric pile has been constructed out of a great number of pairs of antimony and bismuth bars. This instrument in connection with a galvanometer makes a most delicate means of determining slight changes of temperature. If one face of a thermopile is exposed to a temperature greater than its own, the needle will move in one direction; if to a temperature lower than its own, the needle will be deflected in the opposite direction. If both faces of the pile are exposed to the same changes of temperature simultaneously, of course no electrical manifestations will occur.

The earth is undoubtedly a great thermal battery that is kept in action by the constant changes of temperature going on at the earth's surface, caused by its rotation every twenty-four hours on its axis. The sun, of course, is at some point heating the earth, which at other points is cooling, making a constant change of potential between different points. If we heat a metal ring at one point a current of electricity will flow around it—especially if it is made of two dissimilar metals—until the heat is equally distributed throughout the ring.

Some years ago, when the Postal Telegraph Company first began operations between New York and Chicago, the writer made observations twice a day for some time of the temperature and direction of the earth-current. The first two wires constructed gave only two ohms resistance to the mile, which facilitated the experiments. I found that in almost every instance the current flowed from the point of higher temperature to the lower. If the temperature in New York were higher at the time of observations than in Chicago the current would flow westward, and if the conditions were reversed the current would be reversed also.